The design of drive chains of motor vehicles with starting elements executed as hydrodynamic torque converters is sufficiently known from practice.
In most cases, dual line converters are used, each of which is equipped with an integrated torque converter lockup clutch. The torque converter lockup clutches are provided in order to reduce or, as the case may be, eliminate hydraulic losses of the hydrodynamic torque converter.
A converter clutch piston provided to actuate the torque converter lockup clutch, which is impinged upon by hydraulic pressure, is generally configured in a dual line converter as a flexible disk whose hub is connected in a torque-proof manner to a turbine of the hydrodynamic torque converter.
When the torque converter lockup clutch is disengaged, the torque converter piston is impinged upon or overflowed by an oil flow coming from an inflow-side of the hydrodynamic torque converter. In order to engage the torque converter lockup clutch, the supply pressure of the hydrodynamic torque converter is essentially reduced or switched to zero so that a static total pressure present in the hydrodynamic torque converter also essentially drops to zero. The torque converter piston, which is tensioned in the engaging direction of the torque converter lockup clutch and which is coated with a friction lining on the side facing away from the hydrodynamic torque converter, is pressed against the housing of the side of the pump of the hydrodynamic torque converter by the spring mechanism of the converter clutch piston during the last-mentioned operating state.
In order to increase the transmissibility of the torque converter lockup clutch and in that way also increase the torque that can be transmitted by the torque converter lockup clutch, the supply pressure of the torque converter lockup clutch is increased. The increase in the supply pressure of the torque converter lockup clutch increases the contact pressure of the converter clutch piston against the housing of the pump side of the hydrodynamic torque converter. In addition, when the torque converter lockup clutch is engaged, the torque converter lockup clutch piston prevents the oil flow through the hydrodynamic torque converter so that the torque converter lockup clutch essentially performs the function of a traditional hydraulic check valve.
In order to avoid the impairment of driving comfort, the torque converter lockup clutch is preferably engaged only in the operating states of the drive chain in which no stimuli resulting from rotational non-uniformity of the drive chain of a combustion engine, which are acoustically perceived by a driver inside the vehicle, are present. In contrast, however, there is an effort to engage the torque converter lockup clutch as quickly as possible in order to reduce the hydraulic power loss occurring during a startup procedure when the torque converter lockup clutch is disengaged.
Among other things, this leads to the necessity of engaging the torque converter lockup clutch, even with high rotational speed differences in the hydrodynamic torque converter, between a turbine wheel and an impeller of the torque converter, which can have such a negative effect on a engaging operation of the torque converter lockup clutch; that the torque converter lockup clutch cannot be engaged in the intended way or not engaged at all.
Among other things, this leads to the supply pressure of the hydrodynamic torque converter not dropping to zero in the intended manner, but being higher or even lower than the supply pressure of the torque converter lockup clutch, subject to the difference in rotational speed between the turbine piston and the impeller. Because the supply side of the hydrodynamic converter is separated from the torque converter by the converter clutch piston when the torque converter lockup clutch is engaged, the hydrodynamic effects occur to a considerably smaller extent compared to the supply side of the torque converter lockup clutch.
The total force components acting on the converter clutch piston are subject to the current operating state. Therefore, in an operating state of the hydrodynamic torque converter where the supply pressure of the torque converter is smaller than the supply pressure of the torque converter lockup clutch, there is the disadvantageous possibility of the torque converter lockup clutch suddenly engaging before the intended point of engagement or snapping shut, as this is called. In addition, however, there is also the possibility that the torque converter lockup clutch will not engage due to the total force components currently acting on the converter clutch piston. This latter control error of the torque converter lockup clutch is more problematic, because this control error predominantly occurs during an engagement operation of the torque converter lockup clutch at higher rotational speeds of the impeller.
An undesirable hydrodynamic effect resulting in a control error in the torque converter lockup clutch is called a pseudo-cavitation, which occurs when a temperature-dependent pressure level is exceeded in the hydrodynamic torque converter. Before this pressure level is exceeded, air dissolved in the hydraulic fluid passing through the hydrodynamic torque converter, is transformed into gas, which disadvantageously leads to air buildup in the torque converter and on the supply side of the hydrodynamic torque converter.
The abovementioned pressure level in the torque converter is then lowered by the supply pressure of the torque converter used to engage the torque converter lockup clutch being gradually lowered to zero. Then, the supply pressure of the torque converter lockup clutch for adjusting the transmissibility of the torque converter lockup clutch is increased to a designated pressure value. In that way, the static pressure in the torque converter, during the period between the point at which the supply pressure of the torque converter falls below a pressure level under which the air dissolved in the hydraulic fluid degasses and the point at which the supply pressure of the torque converter lockup clutch falls below the pressure level is decreased in such a way that air accumulates in the hydrodynamic torque converter, which forces the fluid present in the hydrodynamic torque converter out of the torque converter.
However, forcing the fluid out disadvantageously leads to a short-term pressure rise in the hydrodynamic torque converter of such nature that a total force component acts temporarily on the converter clutch piston, which causes the torque converter lockup clutch to engage, whereby essentially no torque can be transferred via the torque converter lockup clutch in this state. During subsequent operation of the torque converter, the supply pressure of the torque converter lockup clutch is successively raised in order to adjust the desired transmissibility of the torque converter lockup clutch. However, before the transmissibility of the torque converter lockup clutch rises, the air volume of the conveyed hydraulic fluid that has accumulated in the torque converter due to pseudo-cavitation must be forced out so that, despite the rise in the supply pressure of the torque converter lockup clutch, there will be a certain dead time in the control of the torque converter lockup clutch before the transmissibility of the torque converter lockup clutch rises.
At the point when the accumulated air has been completely forced out of the torque converter, due to the rise in pressure of the supply pressure of the torque converter lockup clutch, the torque converter lockup clutch suddenly has a transmissibility at which at least a torque applied to it is in part transmitted so that there is a noticeable and uncomfortable jolt for the driver of a motor vehicle due to a discontinuity in the torque transmission in the area of the torque converter lockup clutch, which results in an overall undesirable and uncomfortable ride quality of the motor vehicle.
Therefore, the task of the present invention is to provide a hydraulic control system for a hydrodynamic torque converter with a controlled torque converter lockup clutch of an automatic transmission, such that greater driving comfort can be achieved.